Math
Percentage Decrease Calculator
Three ways to work with a percentage decrease: take a percent off a starting value, find the percent decrease between an original and a final value, or reverse a decrease to recover the original from what is left.
Each mode shows the multiplier, the amount removed, the percent remaining, and the formula with your numbers.
The starting amount.
For example 25 for a 25% decrease.
Try an example
Result
Result
—
- Amount removed
- —
- Multiplier
- —
- Percent remaining
- —
- Original value
- —
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Study path
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Percentage decrease formulas
A percent decrease compares how much a value dropped with where it started. The formulas below are one relationship rearranged for the missing quantity. The percent remaining is 100 minus the percent decrease, which is why the multiplier is (1 - p/100).
| Question | Numbers | Answer |
|---|---|---|
| Decrease 80 by 25% | 80 × 0.75 | 60 |
| Percent decrease from 80 to 60 | (80 - 60) / 80 × 100 | 25% |
| Original before a 25% decrease to 60 | 60 ÷ 0.75 | 80 |
| Decrease 120 by 100% | 120 × 0 | 0 |
Reversing a discount
If a sale price is $60 after 25% off, the original was 60 ÷ 0.75 = $80, not 60 + 25% = $75. The discount was taken from the original, so you must divide by the multiplier rather than add the percent back. This is the most common percent mistake in shopping and finance.
How to use it
- Pick the mode: apply a decrease, find the percent between two values, or recover the original.
- Enter the two known numbers.
- Read the result together with the multiplier, the amount removed, and the percent remaining.
- Open Show the work to see the formula with your values substituted.
How to read the answer
In forward mode the result is what is left after the decrease. In find-percent mode a positive percent is a decrease and a negative one means the value actually grew. In reverse mode the result is the value before the decrease, and the check line confirms it.
Common mistakes and edge cases
- Divide by the original value, not the final one, when finding a percent decrease.
- To reverse a decrease, divide by (1 - p/100); do not add the percent back to the final value.
- A 100% decrease leaves 0, and the original cannot be recovered from that.
- A decrease above 100% makes the value negative, which only makes sense for quantities that can go below zero.
- Percent decrease from 0 is undefined.
Worked examples
Decrease 80 by 25%
Forward
Result
60
Percent decrease from 80 to 60
Find the percent
Result
25%
Original before 25% off to 60
Reverse
Result
80
Full decrease
Decrease 120 by 100%
Result
0
Value grew instead
From 60 to 80 is negative
Result
-33.3333%
Decimal values
Decrease 19.99 by 15%
Result
16.9915
Cannot reverse 100% off
Final 0 after 100%
Result
Error
Undefined from zero
Original 0
Result
Error
Frequently asked questions
How do you calculate percent decrease?+
Subtract the final value from the original, divide by the original, and multiply by 100. From 80 to 60: (80 - 60) / 80 × 100 = 25%.
How do I decrease a number by a percentage?+
Multiply by (1 - percent/100). To take 25% off 80, multiply by 0.75 to get 60.
How do I find the original price before a discount?+
Divide the sale price by (1 - discount/100). A $60 price after 25% off was 60 ÷ 0.75 = $80.
Why can't I add the percent back to reverse a decrease?+
The percent was taken from the larger original value, so adding the same percent of the smaller final value comes up short. Divide by the multiplier instead.
What does a percent decrease over 100% mean?+
The value dropped past zero and became negative. That can happen with temperatures or account balances but not with prices or counts.
Is percent decrease the same as percent change?+
Percent change is signed: negative for a decrease. Percent decrease reports the drop as a positive number. A -25% change is a 25% decrease.
About this calculator
- Written by
- mathcheck editorial team
- Last reviewed
- September 4, 2026
Method
- Uses the values entered by the user and stable formulas documented on the page.
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Last updated: September 4, 2026